Optical tomographic image acquisition apparatus and method of acquiring optical tomographic image which adjusts reference position that acquires optical tomographic image based on sheath interference signal

ABSTRACT

An optical tomographic image acquisition apparatus includes a light source unit emitting light; a light dividing device dividing the light from the unit into measurement light and reference light; a projecting device arranged inside a tubular sheath to project the measurement light onto an object; a combining device combining reflected light reflected from the object or the sheath and the reference light; an interference light detecting device detecting interference light between the reflected light and the reference light; an interference signal acquiring device acquiring an interference signal; a tomographic image acquiring device acquiring an optical tomographic image; and a reference position adjustment section detecting a sheath interference signal that is an interference signal of the reflected light reflected from the sheath from the interference signals and adjusts an optical tomographic image reference position as a reference to acquire the optical tomographic image based on the detected sheath interference signal.

The present application is a Continuation Application of U.S. patentapplication Ser. No. 12/564,721, filed on Sep. 22, 2009, which is basedon and claims priority from Japanese patent application No. 2008-246309,filed on Sep. 25, 2008, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical tomographic imageacquisition apparatus and a method of acquiring an optical tomographicimage, and more particularly, to a technique for adjusting a referenceposition to acquire an optical tomographic image.

2. Description of the Related Art

Japanese Patent Application Laid-Open No. 2008-86414 discloses atechnique for adjusting the optical path length of reference light bymoving a reflection mirror of an optical path length changing devicesuch that the optical path length of measurement light and reflectedlight matches that of the reference light at a window incident point ofan optical probe as a reference point.

SUMMARY OF THE INVENTION

In the technique disclosed in Japanese Patent Application Laid-Open No.2008-86414, however, an interference signal may be obtained at anoptical lens system or the like within the optical probe other than aninterference signal at the window incident point of the optical probe,and the window incident point of the optical probe as the referencepoint may not be detected depending on the intensity of the interferencesignal. Thus, an optical tomographic image reference position as areference to acquire an optical tomographic image may not beappropriately set.

The present invention has been made in view of such circumstances, andit is an object of the present invention to provide an opticaltomographic image acquisition apparatus and a method of acquiring anoptical tomographic image, which can appropriately set an opticaltomographic image reference position as a reference to acquire anoptical tomographic image.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a first aspect of the presentinvention is an optical tomographic image acquisition apparatusincluding: a light source unit which emits light; a light dividingdevice which divides the light emitted from the light source unit intomeasurement light and reference light; a projecting device which isarranged inside a tubular sheath having translucency to project themeasurement light divided by the light dividing device onto an object tobe measured outside the sheath; a combining device which combinesreflected light reflected from the object to be measured or the sheathand the reference light; an interference light detecting device whichdetects interference light between the reflected light and the referencelight combined by the combining device; an interference signal acquiringdevice which acquires an interference signal at each frequency byperforming frequency analysis on the interference light detected by theinterference light detecting device; a tomographic image acquiringdevice which acquires an optical tomographic image at each scanningposition of the object to be measured based on the interference signalsacquired by the interference signal acquiring device; and a referenceposition adjustment section which detects a sheath interference signalthat is an interference signal of the reflected light reflected from thesheath from the interference signals acquired by the interference signalacquiring device and adjusts an optical tomographic image referenceposition as a reference to acquire the optical tomographic image basedon the detected sheath interference signal.

With the present invention, since the optical tomographic imagereference position is adjusted based on the detected sheath interferencesignal, the optical tomographic image reference position can beappropriately adjusted.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a second aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to the first aspect, wherein the sheath interference signalincludes a sheath inner peripheral surface interference signal that isan interference signal of reflected light reflected from an innerperipheral surface of the sheath and a sheath outer peripheral surfaceinterference signal that is an interference signal of reflected lightreflected from an outer peripheral surface of the sheath, and thereference position adjustment section detects an intensity of the sheathinner peripheral surface interference signal, an intensity of the sheathouter peripheral surface interference signal, and an interval between afrequency of the interference light when the sheath inner peripheralsurface interference signal is detected and a frequency of theinterference light when the sheath outer peripheral surface interferencesignal is detected, to detect the sheath interference signal from theinterference signals.

With the present invention, the sheath interference signal can bereliably detected, and the optical tomographic image reference positioncan be appropriately adjusted based on the sheath interference signal.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a third aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to the first or second aspect, further including an opticalpath length changing device which changes an optical path length of thereference light by moving a reflection member which reflects thereference light, wherein the reference position adjustment sectionshifts the frequency of the interference light when the sheath innerperipheral surface interference signal is detected and the frequency ofthe interference light when the sheath outer peripheral surfaceinterference signal is detected in one direction by moving thereflection member in one direction, to detect the sheath interferencesignal from the interference signals.

With the present invention, the sheath interference signal can be morereliably detected, and the optical tomographic image reference positioncan be appropriately adjusted based on the sheath interference signal.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a fourth aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to the third aspect, further including a storage device whichstores the sheath interference signal frequency detected by thereference position adjustment section, wherein the reference positionadjustment section determines an initial position of the reflectionmember based on the sheath interference signal frequency previouslydetected and stored in the storage device.

With the present invention, the sheath interference signal can be morequickly and reliably detected.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a fifth aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to any one of the second to fourth aspects, further includinga frequency setting device which can set the frequency of theinterference light when the sheath outer peripheral surface interferencesignal is detected to a predetermined value.

With the present invention, the optical tomographic image referenceposition can be freely set.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a sixth aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to any one of the second to fifth aspects, wherein thereference position adjustment section adjusts the optical tomographicimage reference position such that the frequency of the interferencelight when the sheath outer peripheral surface interference signal isdetected is 3 MHz to 8 MHz.

In order to achieve the above object, an optical tomographic imageacquisition apparatus according to a seventh aspect of the presentinvention is the optical tomographic image acquisition apparatusaccording to any one of the second to fifth aspects, wherein thereference position adjustment section adjusts the optical tomographicimage reference position such that the frequency of the interferencelight when the sheath outer peripheral surface interference signal isdetected is 15 MHz to 20 MHz.

In order to achieve the above object, a method of acquiring an opticaltomographic image according to an eighth aspect of the present inventionis a method of acquiring an optical tomographic image including thesteps of: dividing light emitted from a light source unit intomeasurement light and reference light; projecting the measurement lightonto an object to be measured outside a tubular sheath havingtranslucency by a projecting device arranged inside the sheath;combining reflected light reflected from the object to be measured orthe sheath and the reference light; detecting interference light betweenthe combined reflected light and reference light; acquiring aninterference signal at each frequency by performing frequency analysison the detected interference light; and acquiring an optical tomographicimage at each scanning position of the object to be measured based onthe acquired interference signals; the method further including thesteps of: detecting a sheath interference signal that is an interferencesignal of the reflected light reflected from the sheath from theacquired interference signals and adjusting an optical tomographic imagereference position as a reference to acquire the optical tomographicimage based on the detected sheath interference signal.

With the present invention, the optical tomographic image referenceposition as the reference to acquire the optical tomographic image canbe appropriately set.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram of an optical tomographic imageacquisition apparatus according to the present invention;

FIG. 2 is a schematic view illustrating an example of a distal endportion of an optical probe shown in FIG. 1;

FIG. 3 is a schematic view illustrating an example of a light sourceunit;

FIG. 4 is a graph illustrating a state in which the wavelength of lightemitted from the light source unit in FIG. 3 is swept;

FIGS. 5A and 5B are a flowchart of zero-path adjustment according to thepresent invention and a graph of an acquired interference signal;

FIG. 6 illustrates a method of determining whether a sheath is detectedwithin a measurable area by using an interference signal; and

FIGS. 7A and 7B illustrate a state of shift of an interference signalwhen a reflection mirror is moved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In the following, a preferred embodiment of the present invention willbe described in detail with reference to the accompanying drawings.

[Description of Optical Tomographic Image Acquisition Apparatus]

FIG. 1 is a system block diagram of an optical tomographic imageacquisition apparatus according to the present invention. An opticaltomographic image acquisition apparatus 1 acquires a tomographic imageof an object to be measured S such as living tissue and cells inside abody cavity by SS-OCT (swept source OCT) measurement by inserting anoptical probe 10 into the body cavity. Although the tomographic image isacquired by the SS-OCT measurement in the present embodiment, thetomographic image may be also acquired by another method such as SD-OCT(spectral domain OCT) measurement.

The optical tomographic image acquisition apparatus 1 includes theoptical probe 10, a rotary adapter 12, a probe scanner 14, aninterferometer 16, a light source unit 18, a signal synchronizationsystem section 20, an A/D conversion and data storage section 22, asignal and image processing section 24, and an image display section 26.

FIG. 2 is a schematic view illustrating an example of the distal endportion of the optical probe 10 shown in FIG. 1. The optical probe 10 inFIG. 2 is inserted into a body cavity through a forceps port, forexample. The optical probe 10 includes a sheath (a probe outer tube) 30,an optical fiber 32, and an optical lens 34. The sheath 30 is a tubularmember having flexibility, and is made of material through whichmeasurement light L1 and reflected light L3 are transmitted. The distalend of the sheath 30 is closed by a cap 36.

The optical fiber 32 guides to the object to be measured S themeasurement light L1 emitted from the interferometer 16, and guides tothe interferometer 16 the reflected light (backscattered light) L3 fromthe object to be measured S obtained by projecting the measurement lightL1 onto the object to be measured S. The optical fiber 32 is housed inthe sheath 30.

A spring 38 is fixed to the outer periphery of the optical fiber 32. Theoptical fiber 32 and the spring 38 are mechanically connected to therotary adapter 12. The optical fiber 32 and the spring 38 are rotated inthe direction of an arrow R1 relative to the sheath 30 by the rotaryadapter 12.

The optical lens 34 has a substantially spherical shape to collect themeasurement light L1 emitted from the optical fiber 32 onto the objectto be measured S. The optical lens 34 also collects the reflected lightL3 from the object to be measured S and causes the collected reflectedlight L3 to enter the optical fiber 32. The focal point of the opticallens 34 is formed at a position apart a distance D from an optical axisLP of the optical fiber 32 in the radial direction of the probe outertube.

The optical lens 34 is fixed to a light emission end portion of theoptical fiber 32 by a fixing member 40. When the optical fiber 32 isrotated in the arrow R1 direction, the optical lens 34 is alsointegrally rotated in the arrow R1 direction. Accordingly, the opticalprobe 10 projects the measurement light L1 emitted from the optical lens34 onto the object to be measured S while performing scanning in thearrow R1 direction (the circumferential direction of the sheath 30).

The operation of the rotary adapter 12 in FIG. 1 which rotates theoptical fiber 32 and the optical lens 34 is controlled by the probescanner 14. The probe scanner 14 controls the rotary adapter 12 torotate in the arrow R1 direction relative to the sheath 30. Whendetermining that the optical fiber 32 is rotated one revolution based ona frame signal from a rotary encoder (not shown) of the rotary adapter12, the probe scanner 14 outputs a rotation clock signal to the signalsynchronization system section 20.

FIG. 3 is a schematic view illustrating an example of the light sourceunit 18. The light source unit 18 emits laser light L while sweeping thewavelength at a constant period. To be more specific, the light sourceunit 18 includes a semiconductor optical amplifier (a semiconductor gainmedium) 42, and an optical fiber FB10. The optical fiber FB10 isconnected to the both ends of the semiconductor optical amplifier 42.

The semiconductor optical amplifier 42 has a function of emitting faintemission light to one end of the optical fiber FB10 when a drive currentis applied thereto, and amplifying the light entering from the other endof the optical fiber FB 0. When the drive current is applied to thesemiconductor optical amplifier 42, an optical resonator is formed bythe semiconductor optical amplifier 42 and the optical fiber FB10, andthe laser light L is thereby emitted to the optical fiber FB10.

A circulator 44 is also connected to the optical fiber FB10. The lightguided through the optical fiber FB10 is partially emitted from thecirculator 44 into an optical fiber FB12. The light emitted from theoptical fiber FB12 is reflected by a collimator lens 46 and a rotatingpolygon mirror 48 and reaches a diffraction grating element 52 via anoptical system 50. The light is dispersed by the diffraction gratingelement 52, and is reflected again by the rotating polygon mirror 48 viathe optical system 50. The light reflected by the rotating polygonmirror 48 enters the optical fiber FB12 again.

The rotating polygon mirror 48 rotates in the direction of an arrow R2.The angle of each reflection plane is thereby varied with respect to theoptical axis of the optical system 50. Accordingly, only the lightwithin a specific wavelength band out of the light dispersed by thediffraction grating element 52 returns to the optical fiber FB12. Thewavelength of the light returning to the optical fiber FB 12 isdetermined by the angle formed between the optical axis of the opticalsystem 50 and the reflection plane. The light having a specificwavelength entering the optical fiber FB12 enters the optical fiber FB10from the circulator 44, and the laser light L having the specificwavelength is emitted into the optical fiber FB1.

Accordingly, when the rotating polygon mirror 48 rotates at a constantspeed in the arrow R2 direction, the wavelength λ of the light enteringthe optical fiber FB10 again is varied at a constant period with time.To be more specific, as shown in FIG. 4, the light source unit 18 emitsthe light L whose wavelength is swept at a constant period T0 (forexample, about 50 μsec) from a minimum sweep wavelength λmin to amaximum sweep wavelength λmax. The light L emitted from the light sourceunit 18 enters the interferometer 16.

Although the case of sweeping the wavelength by rotating the polygonmirror is described as the light source unit 18, the light L may be alsoemitted while sweeping the wavelength at a constant period by using aknown technique such as an ASE light source unit.

The interferometer 16 shown in FIG. 1 is a Mach-Zehnder interferometer,and is formed with various optical components being housed in a housing.The interferometer 16 includes a light dividing device 56 which dividesthe light L emitted from the light source unit 18 into the measurementlight L1 and reference light L2, a combining device 58 which combinesthe reflected light L3 from the object to be measured S obtained byprojecting the measurement light L1 divided by the light dividing device56 onto the object to be measured S, and the reference light L2, aninterference signal acquiring device 60 which acquires an interferencesignal by detecting interference light L4 between the reflected light L3and the reference light L2 combined by the combining device 58, and azero-path adjustment section 61 which adjusts an optical tomographicimage reference position as a reference to acquire an opticaltomographic image described later.

The interferometer 16 and the light source unit 18 are connected to eachother by using an APC (angled physical contact) connector. By using theAPC connector, returning light reflected from the connection end surfaceof the optical connector (the optical fiber) is reduced to a maximumextent, and image degradation of a tomographic image P can be prevented.

The light dividing device 56 has a 2×2 optical fiber coupler, forexample. The light dividing device 56 divides the light L guided throughthe optical fiber FB1 from the light source unit 18 into the measurementlight L1 and the reference light L2. Here, the light dividing device 56divides the light L such that the ratio of the measurement light L1 tothe reference light L2 is 99:1, for example. Two optical fibers FB2 andFB3 are respectively optically connected to the light dividing device56. The divided measurement light L1 enters the optical fiber FB2, andthe reference light L2 enters the optical fiber FB3.

An optical circulator 62 is connected to the optical fiber FB2. Opticalfibers FB4 and FB5 are respectively connected to the optical circulator62. The optical probe 10 which guides the measurement light L1 to theobject to be measured S is connected to the optical fiber FB4. Themeasurement light L1 emitted from the optical circulator 62 is guided tothe optical probe 10 through the optical fiber FB4 and is projected ontothe object to be measured S.

The reflected light L3 reflected from the object to be measured S entersthe optical circulator 62 via the optical fiber FB4 and is emitted intothe optical fiber FB5 from the optical circulator 62. The optical fiberFB4 and the optical probe 10 are connected to each other by an APC(angled physical contact) connector. Returning light reflected from theconnection end surface of the optical connector (the optical fiber) isthereby reduced to a maximum extent, and the image degradation of thetomographic image P can be prevented.

Meanwhile, an optical circulator 64 is connected to the optical fiberFB3. Optical fibers FB6 and FB7 are respectively connected to theoptical circulator 64. An optical path length adjustment device 66 whichchanges the optical path length of the reference light L2 to adjust anacquisition area of the tomographic image is connected to the opticalfiber FB6. The optical path length adjustment device 66 includes anoptical path length coarse adjustment optical fiber 68 which coarselyadjusts the optical path length, and an optical path length fineadjustment device 70 which finely adjusts the optical path length.

One end of the optical path length coarse adjustment optical fiber 68 isdetachably connected to the optical fiber FB6. The other end thereof isdetachably connected to the optical path length fine adjustment device70. A plurality of optical path length coarse adjustment optical fibershaving various lengths are prepared in advance as the optical pathlength coarse adjustment optical fiber 68. One of the optical pathlength coarse adjustment optical fibers 68 having an appropriate lengthis connected as needed.

The optical path length coarse adjustment optical fiber 68 is connectedto the optical fiber FB6 and the optical path length fine adjustmentdevice 70 by APC (angled physical contact) connectors. Returning lightreflected from the connection end surface of the optical connector (theoptical fiber) is thereby reduced to a maximum extent, and the imagedegradation of the tomographic image P can be prevented.

The optical path length fine adjustment device 70 includes a collimatorlens 72, a reflection minor 74, and an optical terminator 76. Thereflection minor 74 reflects the reference light L2 emitted from theoptical path length coarse adjustment optical fiber 68 toward theoptical terminator 76, and reflects the reference light L2 reflectedfrom the optical terminator 76 toward the optical path length coarseadjustment optical fiber 68 again.

The reflection minor 74 is fixed on a movable stage (not shown), and ismoved by a mirror moving device in the optical axis direction of thereference light L2 (the direction of an arrow A), thereby changing theoptical path length of the reference light L2. A doctor or the likeoperates an operation section (not shown) to allow the movable stage tomove the reflection minor 74 in the arrow A direction.

A polarization controller 78 is optically connected to the optical fiberFB7. The polarization controller 78 can rotate the polarizationdirection of the reference light L2.

The polarization controller 78 adjusts the polarization direction bybeing operated by a doctor or the like. For example, by operating thepolarization controller 78 such that the polarization direction of thereference light L2 corresponds to that of the reflected light L3 whenthe reference light L2 and the reflected light L3 are combined in thecombining device 58, the tomographic image can be adjusted to be clear.

The combining device 58 has a 2×2 optical fiber coupler. The combiningdevice 58 combines the reflected light L3 guided through the opticalfiber FB5 and the reference light L2 guided through the optical fiberFB7. To be more specific, the combining device 58 branches the reflectedlight L3 guided through the optical fiber FB5 into two optical fibersFB8 and FB9, and the reference light L2 guided through the optical fiberFB7 into the optical fibers FB8 and FB9.

Accordingly, the reflected light L3 and the reference light L2 arecombined respectively in the optical fibers FB8 and FB9. Firstinterference light L4 a is guided through the optical fiber FB8, andsecond interference light L4 b is guided through the optical fiber FB9.That is, the combining device 58 also functions as a light branchingdevice 59 which branches the interference light L4 between the reflectedlight L3 and the reference light L2 into the interference light L4 a andthe interference light L4 b.

The interference signal acquiring device 60 includes a first lightdetection section 80 which detects the first interference light L4 a, asecond light detection section 82 which detects the second interferencelight L4 b, and a difference amplifier 84 which outputs a differencebetween the first interference light L4 a detected by the first lightdetection section 80 and the second interference light L4 b detected bythe second light detection section 82 as an interference signal.

Each of the light detection sections 80 and 82 has a photodiode or thelike. The light detection sections 80 and 82 photoelectrically convertand input the incident interference light L4 a and interference light L4b to the difference amplifier 84, respectively. The difference amplifier84 amplifies the difference between the interference light L4 a andinterference light L4 b and outputs the difference as the interferencesignal.

As described above, the interference light L4 a and interference lightL4 b are detected by balanced detection by the difference amplifier 84.Accordingly, in-phase light noise other than the interference signal canbe eliminated while amplifying and outputting the interference signal,and the image quality of the tomographic image P can be therebyimproved.

An A/D conversion and data storage section 22 is located in thedownstream of the difference amplifier 84 of the interference signalacquiring device 60. The A/D conversion and data storage section 22 isconnected to the zero-path adjustment section 61 via a signal and imageprocessing section 24. The zero-path adjustment section 61 is connectednot only to the signal and image processing section 24 but also to themovable stage which moves the reflection mirror 74 of the optical pathlength adjustment device 70 and the probe scanner 14. The zero-pathadjustment section 61 controls the optical path length adjustment device70 so that a frequency of an interference signal which is outputted fromthe interference signal acquiring device 60, subjected to A/Dconversion, and then subjected to signal processing on the signal andimage processing section 24 becomes a predetermined frequency.

An operation example of the optical tomographic image acquisitionapparatus 1 will be described with reference to FIGS. 1 to 4. First, thelight source unit 18 emits a light flux whose wavelength is swept at aconstant period within a predetermined wavelength band. The light Lenters the interferometer 16. The light L is divided into themeasurement light L1 and the reference light L2 by the light dividingdevice 56 of the interferometer 16. The measurement light L1 is emittedinto the optical fiber FB2, and the reference light L2 is emitted intothe optical fiber FB3.

The measurement light L1 is guided through the optical circulator 62,the optical fiber FB4 and the optical probe 10 to be projected onto theobject to be measured S. The reflected light L3 reflected from eachdepth position z of the object to be measured S and backscattered lightenter the optical probe 10 again. The reflected light L3 enters thecombining device 58 through the optical probe 10, the optical circulator62, and the optical fiber FB5.

On the other hand, the reference light L2 enters the optical path lengthadjustment device 66 through the optical fiber FB3, the opticalcirculator 64, and the optical fiber FB6. The reference light L2 whoseoptical path length has been adjusted by the optical path lengthadjustment device 66 is guided through the optical fiber FB6, theoptical circulator 64, the polarization controller 78, and the opticalfiber FB7 to enter the combining device 58.

The reflected light L3 and the reference light L2 are combined in thecombining device 58. The interference light L4 between the combinedreflected light L3 and reference light L2 is branched in the combiningdevice 58 (the light branching device 59), and the interference light L4a and interference light L4 b are respectively emitted into the opticalfibers FB8 and FB9. The interference light L4 a and interference lightL4 b respectively guided through the optical fibers FBS and FB9 aredetected by balanced detection by the interference signal acquiringdevice 60.

The interference light L4 detected by balanced detection by theinterference signal acquiring device 60 is output as the interferencesignal to the A/D conversion and data storage section 22. Theinterference signal is then A/D converted in the A/D conversion and datastorage section 22.

The signal and image processing section 24 performs signal conversionprocessing on the interference signals corresponding to a single linesuch that the interference signals are at regular intervals with respectto a wavenumber k. After that, spectral analysis is performed on theinterference signals to acquire tomographic data (reflectance)respectively from the interference signals as tomographic data. Theacquired tomographic data is accumulated for n lines in the scanningdirection (the direction of the arrow R1) of the measurement light L1.

When the rotation clock signal from the probe scanner 14 is detected,one optical tomographic image is generated by using the accumulatedplurality of pieces of tomographic data. The image quality of thegenerated optical tomographic image is corrected, and the opticaltomographic image whose image quality has been corrected is displayed onthe image display section 26 in FIG. 1.

[Description of Zero-Path Adjustment]

Next, the zero-path adjustment for adjusting the optical tomographicimage reference position as the reference to acquire the opticaltomographic image in the aforementioned optical tomographic imageacquisition apparatus 1 will be described. The flowing zero-pathadjustment can be performed during measurement as well as when theoptical probe 10 is replaced or the optical tomographic imageacquisition apparatus 1 is started up.

FIG. 5A is a flowchart of the zero-path adjustment according to thepresent invention. As shown in FIG. 5A, first, a zero-path adjustmentbutton (not shown) arranged on the operation section of the apparatus ispressed (step S1).

It is checked whether the optical probe 10 is connected to the rotaryadapter 12 (step S2). When the optical probe 10 is not connected to therotary adapter 12, the optical probe 10 is connected thereto (step S3),and the zero-path adjustment button is pressed (step S1).

On the other hand, when the optical probe 10 is connected to the rotaryadapter 12 in step S2, the rotation operation of the optical probe 10 isstarted (step S4).

Subsequently, the reflection mirror 74 of the optical path length fineadjustment device 70 as a delay device is moved to an initial position(step S5). An interference signal at the initial position is detected bythe interference signal acquiring device 60 (step S6). A memory (notshown) may store the position of the reflection mirror 74 after theprevious zero-path adjustment, to employ a position apart apredetermined distance from the stored position of the reflection minor74 as the initial position.

The interference signal as shown in FIG. 5B in which the horizontal axisrepresents the frequency (depth) of the interference light and thevertical axis represents the value of the interference signal isacquired here. As shown in FIG. 5B, the interference signal has two peakvalues at given frequencies (given depths) within a measurable area.

However, the interference signal may have a peak value due to reflectedlight from the optical lens 34 in the optical probe 10, or reflectedlight within a body cavity when the optical probe 10 is inserted intothe body cavity, for example.

Thus, at this point in time, it is not clear whether the two peak valuesof the interference signal are caused by an interference signal ofreflected light from an inner peripheral surface 30 a and an outerperipheral surface 30 b of the sheath 30.

In the following steps, it is determined whether the two peak values ofthe interference signal are caused by the interference signal of thereflected light from the inner peripheral surface 30 a and the outerperipheral surface 30 b of the sheath 30, that is, whether the sheath 30is detected within the measurable area.

First, the interference signal acquiring device 60 acquires theinterference signals while moving the reflection mirror 74 from theinitial position. It is then checked whether the reflection mirror 74has been moved to a predetermined end position (step S7). When thereflection mirror 74 has not reached the predetermined end position, thereflection mirror 74 is further moved (step S8). The reflection mirror74 is continuously or intermittently moved. Alternatively, thereflection mirror 74 may be moved with large steps to a position wherethe interference signal can be acquired, and then, moved with smallsteps.

Subsequently, when the reflection mirror 74 has reached thepredetermined end position, it is determined whether the sheath 30 isdetected within the measurable area by using the interference signalsacquired with the reflection mirror 74 being moved (step S9). A specificdetermination method will be described below.

FIG. 6 illustrates a method of determining whether the sheath 30 isdetected within the measurable area by using the interference signals.

As shown in FIG. 6, first, the interference signal is cut off at apredetermined threshold, and the cut-off peak values of the interferencesignal are detected. Here, two peak values of the interference signalare detected. One of the peak values selected from the two peak valuesof the interference signal is detected. It is then detected whether thesecond peak value exists at a frequency apart by a width D1corresponding to the thickness of the sheath 30 (a distance between theinner peripheral surface 30 a and the outer peripheral surface 30 b)from the frequency at which the first peak value is detected.

When the second peak value is detected to exist at the above frequency,the two peak values of the interference signal are highly likely to bethe values of the interference signal of the reflected light from theinner peripheral surface 30 a and the outer peripheral surface 30 b ofthe sheath 30.

By further checking a state of movement of the reflection mirror 74 andshift of the interference signal, it is finally determined whether thesheath 30 is detected within the measurable area.

To be more specific, it is determined as described below.

FIGS. 7A and 7B illustrate the state of shift of the interference signalwhen the reflection mirror 74 is moved.

In a case where the two peak values of the interference signal shown inFIG. 7A are caused by the interference signal of the reflected lightfrom the inner peripheral surface 30 a and the outer peripheral surface30 b of the sheath 30, the two peak values of the interference signalare shifted in a direction in which the frequency becomes higher asshown in FIG. 7B when the reflection mirror 74 is moved from the initialposition to the predetermined end position in a direction in which thereference light L2 has a longer optical path length.

Thus, when it is confirmed that the two peak values of the interferencesignal are shifted in the direction in which the frequency becomeshigher as shown in FIG. 7B by moving the reflection mirror 74 from theinitial position, it is determined that the two peak values of theinterference signal are caused by the interference signal of thereflected light from the inner peripheral surface 30 a and the outerperipheral surface 30 b of the sheath 30.

Accordingly, the interference signal of the sheath 30 can be stablydetected within the measurable area.

If the interference signal of the sheath 30 cannot be detected withinthe measurable area in step S9, an error is displayed on the displaysection arranged on the operation section of the apparatus (step S10).

When the interference signal of the sheath 30 can be detected within themeasurable area in step S9, the position to which the reflection mirror74 is to be moved is calculated such that the optical tomographic imagereference position is set based on the position (frequency, depth) ofthe detected sheath 30 (step S11).

Following methods may be employed as a method of setting the opticaltomographic image reference position based on the position of thedetected sheath 30.

As a first method, a depth to be measured is represented by Z on theassumption that the object to be measured S contacts the outerperipheral surface 30 b of the sheath 30. The position of the reflectionmirror 74 is adjusted so as to acquire an interference signal having ahigh frequency at the outer peripheral surface 30 b of the sheath 30 bysetting the optical tomographic image reference position at a positionapart a distance of Z from the outer peripheral surface 30 b of thesheath 30.

A following expression is calculated when the frequency and thewavenumber at the outer peripheral surface 30 b of the sheath 30 arerespectively represented by f and k, for example.

$\begin{matrix}{f = {\left( \frac{Z}{\pi} \right) \times \left( \frac{\mathbb{d}k}{\mathbb{d}t} \right)}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

As a specific example, when the sweep frequency of the light source is20 kHz (a duty ratio: 65%), the sweep wavelength interval is 150 nm (acentral wavelength: 1.31 μm), and Z is 3.4 mm, the position of thereflection mirror 74 is calculated and adjusted such that the frequencyf at the outer peripheral surface 30 b of the sheath 30 is 18.3 MHz.Accordingly, the optical tomographic image reference position is set ata position apart a distance of Z=3.4 mm from the outer peripheralsurface 30 b of the sheath 30.

In the first method, the frequency fat the outer peripheral surface 30 bof the sheath 30 is preferably 15 MHz to 20 MHz.

As a second method, the position of the reflection minor 74 is adjustedso as to acquire an interference signal having a low frequency at theouter peripheral surface 30 b of the sheath 30 and an interferencesignal having a high frequency at a predetermined depth position of theobject to be measured S by setting the optical tomographic imagereference position at a position between the optical axis of the opticalfiber 32 within the optical probe 10 and the inner peripheral surface 30a of the sheath 30.

A following expression is calculated when the distance between theoptical axis of the optical fiber 32 and the inner peripheral surface 30a of the sheath 30 is represented by d and the optical tomographic imagereference position is set at the position of the optical axis LP (seeFIG. 2) of the optical fiber 32 in the optical probe 10, for example.

$\begin{matrix}{f = {\left( \frac{\left( {d\text{/}4} \right)}{\pi} \right) \times \left( \frac{\mathbb{d}k}{\mathbb{d}t} \right)}} & \left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack\end{matrix}$

As a specific example, when the sweep frequency of the light source is20 kHz (a duty ratio: 65%) and the sweep wavelength interval is 150 nm(a central wavelength: 1.31 μm) in a similar manner to the above on theassumption that d is 1.25 mm, the position of the reflection minor 74 iscalculated and adjusted such that the frequency fat the outer peripheralsurface 30 b of the sheath 30 is 6.75 MHz. Accordingly, the opticaltomographic image reference position is set at the position of theoptical axis LP of the optical fiber 32 in the optical probe 10.

In the second method, the frequency fat the outer peripheral surface 30b of the sheath 30 is preferably 3 MHz to 8 MHz.

As the method of setting the optical tomographic image referenceposition based on the detected position of the sheath 30, the opticaltomographic image reference position may be also set by manuallyinputting the frequency fat the outer peripheral surface 30 b of thesheath 30 through an input device (such as a touch panel, not shown)other than by the calculation as described above.

Accordingly, the frequency f of the interference signal at the outerperipheral surface 30 b of the sheath 30 can be freely changed by usingthe input device.

As the method of setting the optical tomographic image referenceposition based on the detected position of the sheath 30, the opticaltomographic image reference position may be also set based on thefrequency f at the outer peripheral surface 30 b of the sheath 30 whichis stored in a memory (not shown) in advance.

Although the optical tomographic image reference position is set basedon the frequency f at the outer peripheral surface 30 b of the sheath 30in the present embodiment, the optical tomographic image referenceposition may be also set based on the frequency at the inner peripheralsurface 30 a of the sheath 30.

The methods described above are employed as the method of setting theoptical tomographic image reference position based on the detectedposition of the sheath 30.

Subsequently, the reflection mirror 74 is moved to the movement positionobtained as described above (step S12), and the zero-path adjustment isterminated (step S13).

As described above, in the present invention, the inner peripheralsurface 30 a and the outer peripheral surface 30 b of the sheath 30 ofthe optical probe 10 can be stably detected within the measurable area.Accordingly, a zero-path position that is the optical tomographic imagereference position as the reference to acquire the optical tomographicimage can be reliably adjusted based on the detected position of theouter peripheral surface 30 b of the sheath 30.

The optical tomographic image acquisition apparatus and the method ofacquiring the optical tomographic image according to the presentinvention are described above in detail. However, it goes without sayingthat the present invention is not limited to the aforementionedembodiment and various modifications and changes may be made thereinwithout departing from the scope of the present invention.

What is claimed is:
 1. An optical tomographic image acquisitionapparatus, comprising: a light source unit which emits light whosewavelength is swept at a constant period time between a minimum sweepwavelength and a maximum sweep wavelength; a light dividing device whichdivides the light emitted from the light source unit into a measurementlight and a reference light; an optical probe comprising a tubularsheath and an optical fiber arranged inside the sheath, the opticalfiber configured to guide the measurement light divided by the lightdividing device to an object to be measured outside the sheath and toguide a reflected light reflected from the object; a combining devicewhich combines the reflected light reflected from the object to bemeasured and the sheath and the reference light; an interference signalacquiring device which detects an interference light between thereflected light and the reference light combined by the combining deviceand acquires the interference light as an interference signal; a signaland image processing section which acquires a tomographic informationfor generating an optical tomographic image at each scanning position ofthe object to be measured by performing signal processing of theinterference signal acquired by the interference signal acquiringdevice; an optical path length adjustment device configured to adjust anoptical path length of the reference light; and a zero-path adjustmentsection which controls the optical path length adjustment device suchthat the interference signal acquired by the interference signalacquiring device becomes a predetermined frequency.
 2. An opticaltomographic image acquisition apparatus, comprising: a light source unitwhich emits light whose wavelength is swept at a constant period timebetween a minimum sweep wavelength and a maximum sweep wavelength; alight dividing device which divides the light emitted from the lightsource unit into a measurement light and a reference light; an opticalprobe comprising a tubular sheath and an optical fiber arranged insidethe sheath, the optical fiber configured to guide the measurement lightdivided by the light dividing device to an object to be measured outsidethe sheath and to guide a reflected light reflected from the object; acombining device which combines the reflected light reflected from theobject to be measured and the sheath and the reference light; aninterference signal acquiring device which detects an interference lightbetween the reflected light and the reference light combined by thecombining device and acquires the interference light as an interferencesignal; a signal and image processing section which acquires atomographic information for generating an optical tomographic image ateach scanning position of the object to be measured by performing signalprocessing of the interference signal acquired by the interferencesignal acquiring device; an optical path length adjustment deviceconfigured to adjust an optical path length of the reference light; anda zero-path adjustment section which controls the optical path lengthadjustment device such that the interference signal acquired by theinterference signal acquiring device becomes a predetermined frequency,wherein the zero-path adjustment section controls the optical pathlength adjustment device so that the interference signal reflected froma surface of the sheath amongst interference signals becomes apredetermined frequency.
 3. A method of acquiring an optical tomographicimage, said method comprising: dividing a light whose wavelength isswept at a constant period time between a minimum sweep wavelength and amaximum sweep wavelength emitted from a light source unit into ameasurement light and a reference light; providing an optical probe thatcomprises a tubular sheath and an optical fiber arranged inside thesheath, the optical fiber configured to guide the measurement lightdivided in the dividing of the light to an object to be measured outsidethe sheath and to guide reflected light reflected from the object;combining a reflected light reflected from the object to be measured andthe sheath and the reference light; acquiring an interference lightbetween the reflected light and the reference light that is combined asan interference light signal; acquiring tomographic information forgenerating an optical tomographic image at each scanning position of theobject to be measured by performing signal processing of theinterference signal acquired by the interference signal acquiringdevice; and adjusting an optical path length of the reference light sothat the acquired interference signal becomes a predetermined frequency.